The present invention relates generally to rotatable members that are able to achieve balanced conditions throughout a range of rotational speeds. The present invention also relates to methods and systems for dynamically balancing rotatable members through the continual determination of out-of-balance forces and motion to thereby take corresponding counter balancing action. The present invention additionally relates to methods and systems in which inertial masses are actively placed within a rotating body in order to cancel rotational imbalances associated with the rotating body thereon. The present invention additionally relates to timely methods and system that extract measured signal components indicative of the balance condition of the rotating system and used to build the rotating system control model as well as influence the course of dynamic balance control.
Mass unbalance in rotating machinery leads to machine vibrations that are synchronous with the rotational speed. These vibrations can lead to excessive wear and to unacceptable levels of noise. Typical imbalances in large rotating machines are on the order of one inch-pound.
It is a common practice to balance a rotatable body by adjusting a distribution of moveable, inertial masses attached to the body. Once certain types of bodies have been balanced in this fashion, they will generally remain in balance only for a limited range of rotational velocities. A tire, for instance, can be balanced once by applying weights to it. This balanced condition will remain until the tire hits a very big bump or the weights are removed. A centrifuge for fluid extraction, however, can change the amount of balance as more fluid is extracted.
Many machines are also configured as free standing spring mass systems in which different components thereof pass through resonance ranges until the machine is out of balance. Additionally, such machines may include a rotating body flexibly located at the end of a shaft rather than fixed to the shaft as in the case of a tire. Thus, moments about a bearing shaft may also be created merely by the weight of the shaft. A flexible shaft rotating at speeds above half of its first critical speed can generally assume significant deformations, which add to the imbalance. This often poses problems in the operation of large turbines and turbo generators.
Machines of this kind usually operate above their first critical speed. As a consequence, machines that are initially balanced at relatively low speeds may tend to vibrate excessively as they approach full operating speed. Additionally, if one balances to an acceptable level rather than to a perfect condition (which can be difficult to measure), the small remaining out of balance will progressively apply force as the speed increases. This increase in force is generally due to the fact that Fxcex1rxcfx892 (note that F represents the xe2x80x9cout-of-balancexe2x80x9d force, r represents the radius of the rotating body and xcfx89 represents its rotational speed).
The mass unbalance distributed along the length of a rotating body may give rise to a rotating force vector at each of the bearings that support the body. In general, the force vectors at respective bearings are not in phase. At each bearing, the rotating force vector may be opposed by a rotating reaction force, which can be transmitted to the bearing supports as noise and vibration.
The purpose of active, dynamic balancing is to shift an inertial mass to the appropriate radial eccentricity and angular position for canceling the net mass unbalance. At the appropriate radial and angular distribution, the inertial mass can generate a rotating centrifugal force vector equal in magnitude and phase to the reaction force referred to above.
Many different types of balancing schemes are known to those skilled in the art. When rotatable objects are not in perfect balance, nonsymmetrical mass distribution creates out-of-balance forces because of the centrifugal forces that result from rotation of the object. Although rotatable objects find use in many different applications, one particular application is a rotating drum of a washing machine.
U.S. Pat. No. 5,561,993, which issued to Elgersma et al. on Oct. 22, 1996 (assigned to the owner of the present application) and is incorporated herein by reference, discloses a self-balancing rotatable apparatus. Elgersma et al. disclosed a method and system for measuring forces and motion via accelerations at various locations in a system. The forces and moments were balanced through the use of a matrix manipulation technique for determining appropriate counterbalance forces located at two axial positions of the rotatable member. The method and system described in Elgersma et al. accounted for possible accelerations of a machine, such as a washing machine, which could not otherwise be accomplished if the motion of the machine were not measured. Such a method and system was operable in association with machines not rigidly attached to immovable objects, such as concrete floors. The algorithm disclosed by Elgersma et al. permits counterbalance forces to be calculated even though a washing machine is located on a moveable floor structure combined with carpet padding and carpets between the washing machine and a rigid support structure.
U.S. Pat. No. 5,561,993 thus described a dynamic balance control algorithm for balancing a centrifuge for fluid extraction. To accomplish such balance control, sensor measurement data may be utilized to assess the immediate balance conditions and determine the course of balance control. Related sensor responses to balance control actions may be modeled to determine the specific future control actions. In making sensor measurements, relevant acceleration and force data may be obtained from sensors on a rotating device with a narrow band pass filtering function that is tunable in real time to the speed of rotation. The band pass function can be accomplished through correlation of the sensor signal with a sinusoid referenced to the position of the rotating device.
By obtaining at least one known position of the rotating device, data can be measured and calculated with respect to that position. Creating two summations correlates measured points: an X summation and a Y summation. The X summation is generally one revolution of data points multiplied by a cosine reference term with respect to the position of the rotating device. The Y summation is generally one revolution of data points multiplied by a sine reference term with respect to the position of the rotating device. For a reasonable filtered or correlated result, two or three revolutions of data can be collected utilizing a direct memory access. A correlation can be then performed on the complete data set. This approach takes a great deal of time, both in process and delay time, while waiting for the device to rotate. This time can be critical because the balance condition is constantly changing. Additionally, variation in rotational speed can impact correlated results, yet a check is not available to validate the correlated result.
Thus, there exists a need for a method and system for implementing a correlation filter function in a manner that reduces computation and process delay times, thereby providing a quality measure of the correlated result. The present invention described herein overcomes these obstacles through the use of specially-indexed sinusoidal reference tables representing a fixed number of data points per signal period, independent of rotational speed, and through the use of quality parameters based on actual versus expected data samples per signal period.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
In accordance with addressing the shortcomings of the prior art, it is one aspect of the present invention to provide methods and systems in which rotatable members can achieve balanced conditions throughout a range of rotational speeds.
It is another aspect of the present invention to provide methods and systems for dynamically balancing rotatable members through the continual determination of out-of-balance forces and motion to thereby take corresponding counter balancing action.
It is still another aspect of the present invention to provide dynamic correlation extensions and improvements for a self-balancing rotatable apparatus.
In accordance with various aspects of the present invention, methods and systems are discussed herein for dynamically balancing a rotating system utilizing filtered sensor signals, wherein the rotating system contains sensors therein. A sine table and a cosine table is created based on a particular number of measured data points per revolution of the rotating device and referenced to the rotational position. Data contained within the sine and cosine tables are then correlated in real time with sensor signals to obtain sensor measurements associated with a balance condition of the rotating device. The dynamically correlated data can be thereby utilized to build a rotating system control model as well as influence the course of dynamic balance control. An error calculation based on a sample size and the number of samples obtained since a last index pulse can be utilized to compute the validity of the data. An index can be associated with the reference sine table and an additional index associated with the cosine reference table. Each of the indexes can be formulated based on an angle per index, which in turn is based on a particular number of measured data points desired per signal period divided by 360 degrees. At least one sine element and at least one cosine element are respectively calculated for an index associated with the sine table and the index associated with the cosine table. The sine and cosine tables can be combined into one table to thereby reduce memory required to dynamically correlate the signal data with the data contained within the sine and cosine tables.